Optical fiber communication systems typically carry a plurality of signals over different paths. These paths may be modified over time as the system is updated or changed. Variations in path length, fiber types and components encountered by optical signals may degrade a signal. Additional conditions and parameters such as temperature, input power and wavelength may also affect the characteristics of the signal.
To maintain quality of service in an optical fiber communication system, bit-error-rate (BER) should be minimized and signal-to-noise ratio (SNR) at the decision time of the decision circuit should be maximized. An optoelectronic receiver linear circuit comprising a photodiode, amplifier and low pass filter provide a transfer function to shape the received signal spectrum. The low pass filter optimizes the transfer function to desired characteristics, typically compensating for the band shape of the photodiode and amplifier to produce a channel shape that minimizes inter-symbol interference (ISI) and BER and maximizes SNR on the input of the decision circuit. Typically each stage of the receiver contributes to the transfer function of the overall post-detection filter. The combination of photodetector and amplifier transfer functions can be used to develop the filter transfer function.
Frequency response functions of the linear circuit components vary with different operational conditions and as a result of manufacturing variations. Operational conditions may include, for example, temperature, time, channel number in wavelength division multiplexing (WDM) systems, noise, etc. An avalanche photodiode (APD) for example, has gain and noise characteristics that generally vary depending on the bias voltage, temperature and age of the device. A typical APD transfer function changes as a function of temperature change as shown in FIG. 1. For a particular amplitude the APD frequency increases as temperature decreases. Lines 102 and 104 in FIG. 1 depict the transfer functions at a low temperature, and high temperature, respectively.
A further example of a linear circuit component whose performance varies according to operating conditions is a transimpedance amplifier (TIA) having automatic gain control (AGC). A TIA gain frequency response typically depends on the input current or supply voltage and can vary over temperature.
Manufacturing variations of components may also affect the linear circuit transfer function. For example, a TIA integrated circuit and optical subassembly (OSA) wire bond may have variations in wire bond length from one component to another and within a component, resulting in the distribution of their parasitic inductance, thereby affecting the transfer function. The effect of the wire bond length distribution on the inductance variation increases if the receiver bit-rate and linear circuit bandwidth increase.
Total noise spectra may also vary as operational conditions change. Different optical transmitters and channels in a dense wavelength division multiplexing (DWDM) system may have different noise spectra. Noise may also vary as the spontaneous emission power generated by the optical amplifier varies.
Because parameters of a transmitter, channel and linear circuit of the receiver are not stable when operational conditions are changed, the input signal and noise spectra may vary. Therefore, a low pass filter with fixed parameters, such as a fixed bandwidth, does not provide optimum channel shaping and noise filtering over the different operational conditions.
If, for example, at an operational condition the filter pass band is too wide, the additional noise power coming to a decision circuit from the optoelectronic receiver degrades SNR, BER and the receiver sensitivity. If the filter bandwidth is too narrow or if the received pulse shape does not satisfy a Nyquist criteria, ISI increases, and BER and sensitivity degrade.
FIG. 2 depicts a prior art optical receiver 200. A power supply 202 and temperature sensor 204 provide a temperature-compensated biasing voltage to photodiode 206 over a temperature range. This temperature-compensated biasing voltage is needed for an APD-type receiver only. PIN-type receivers usually do not require such biasing. A photocurrent sensor 208 may also be included to provide further adjustment to the biasing voltage, or to provide monitoring of an optical system operation. An optical input light (OIL) sensor 210 provides an electrical signal from an additional output of amplifier 212 that is proportional to the power of the photodetector input optical signal. Amplifier 212 provides a signal to low pass filter 214 which provides a filtered signal to decision circuit 216. A quasi-BER detector 218 provides a signal indicating BER of the output electrical signal from the decision circuit.
Limitations of conventional fiberoptic receivers have been addressed by utilizing a microcontroller to dynamically monitor and control parameters of an optoelectronic device and other receiver module components. During calibration procedures of the receiver, the components of the receiver module are characterized over a defined operating temperature and voltage supply range. Characteristic data and/or curves defining these operational control and monitoring functions over the range of operating conditions are stored in non-volatile memory. During operation of the receiver the module's operational parameters are controlled based on the current operating conditions.
Another approach to improve optical receiver performance includes providing a linear channel section with an active equalizer controlled by the photo current of an input network. By utilizing the inverse relationship between the impedance of the front end and the input signal current and an appropriate scaling of the current transfer gain, the equalizer frequency response is made to track changes in the front end frequency response.
Existing techniques as described above do not adequately address the transfer function variations of the linear circuit at different operational conditions. Such variations change the shape of pulses, increase ISI and decrease SNR on the input to the decision circuit. Furthermore, BER and sensitivity of the receiver may be degraded and may limit the operating conditions under which the circuit may be used. At ultra high bit-rates even a small signal distortion may induce significant degradation of system performance. Accordingly, there is a need for an optical receiver having a variable transfer function to compensate for operating conditions.
Such a system may reduce adverse effects due to manufacturing variations, and may negate the need for extensive testing and selection of receiver components which is costly and time consuming for the mass production of receivers.
Furthermore, prior art systems do not control the shape of the frequency response of the linear circuit of the receiver, and therefore, high sensitivity can not be reached at different operational conditions.
Additionally, an approach utilizing an equalizer as described above is limited to use in receivers with AGC. The equalizer, which is a high pass filter is controlled by a photocurrent only. Use of the equalizer in the linear circuit of the fiberoptic receiver typically causes a sensitivity penalty due to noise from stages following the equalizer or reduction in dynamic range due to saturation of the amplifier preceding the equalizer. Therefore, little or no sensitivity improvement is provided by such a design.